The present invention relates to an electronic delay detonator for delayed detonation after the lapse of a predetermined delay time in response to a pulse-like ignition input voltage.
The conventional electric delay detonator specified in JIS K4807 (JIS is an abbreviation of Japanese Industrial Standard) is such that delay powder is arranged between an electric ignition device (platinum wire) and a charge to deal the detonation time. The control of the mixing of this delay powder and management of the charge amount thereof are very troublesome on the one hand, and the precision of the delay time is generally low on the other hand. In recent years, with the improved civil engineering technologies, there has been an increasing demand for an improved time accuracy of the delay detonator. So far, the accuracy of the electric delay detonator with delay powder has been limited to .+-.3 to 4% of a set delay time.
In view of this, some researchers have suggested an electronic delay detonator with an electrical circuit which is low in production cost and high in time accuracy. For technologies related to the electronic delay detonator, reference is made to Japanese Patent Laid-Open No. 43454/79 laid open on Apr. 6, 1979, Japanese Patent Laid-Open No. 142496/82 laid open on Sept. 3, 1982, Japanese Patent Laid-Open No. 142498/82 laid open on Sept. 3, 1982, and U.S. Pat. No. 4,240,350.
These electronic delay detonators are roughly classified into two types; an analog system comprising a time delay device including a series connection of a resistor and a capacitor in which the voltage across the capacitor is utilized, and a digital system comprising a CR oscillator circuit or a crystal oscillator circuit and a counter so that the pulses generated by the oscillator circuit are counted to attain a predetermined delay time.
The former detonator generally comprises a capacitor for storing electrical energy, a thyristor, an electrical ignition device (such as platinum wire) connected in series with the storage capacitor through the thyristor, a series connection of a resistor and a capacitor for driving the thyristor with a predetermined delay time after application of the electrical energy to the storage capacitor, a thyristor trigger device inserted between the gate of the thyristor and the time-delaying capacitor for applying the electrical energy stored in the time-delaying capacitor to the gate of the thyristor when the voltage across the time-delaying capacitor reaches a predetermined level, and a constant voltage circuit connected across the storage capacitor for applying a constant voltage across the time-delaying resistor and the capacitor regardless of the input voltage of the storage capacitor.
The electronic delay detonator with analog voltage, however, has different delay times depending on the voltage applied to the storage capacitor and temperature changes, and has not any conspicuous advantage as compared with the detonator with delay powder. Due to these facts and variations in electrical characteristics of parts to be used, it is difficult to produce such analog type detonators of practical use on a mass production basis.
Generally, the error of charge-discharge cycle under transient conditions increases with the capacitor's capacity. If this error is to be reduced to a minimum, the capacitor's capacity should desirably be minimized. In the electronic delay detonator with analog voltage, however, the time delaying capacitor is used to determine the delay time, and also used to fire the thyristor with the electrical energy stored therein. It is, therefore, impossible to use a capacitor of a capacity smaller than a predetermined value, resulting in the problem of impossibility of error reduction and the problem of the unavailability of a wide setting range of delay time.
In the digital system aimed at high accuracy of delay time, on the other hand, it is common practice to use an oscillator circuit including an oscillator such as a crystal oscillator or a ceramic oscillator or a CR oscillator circuit whereby the oscillation output is frequency-divided to effect accurate counting of the time. The detonator with a CR oscillator has the problem of insufficient accuracy of the oscillation frequency, while the oscillator circuit including a crystal oscillator or the like involves the following inconveniences in the delaying operation of the electronic delay detonator and is not of practical value. Specifically, the oscillation of a crystal oscillator or the like uses the vibrations by the mechanical displacement of a solid, and accordingly, it takes a long time such as several hundred milliseconds for the low-frequency oscillator or several tens of milliseconds for the high-frequency oscillator before a predetermined vibration frequency is established.
When one tries to delay the time accurately after the application of electrical ignition energy, therefore, the delay means using the crystal oscillator develops an error in delay time due to this initial unstable period of time, making it impossible to use the detonator reliably for delay blasting. If this initial unstable period is to be eliminated, it is necessary to excite the crystal oscillator circuit with another power supply in advance. In delay firing or ignition of detonators where electrical ignition energy is applied to all the detonators at a time for sequential detonations, however, it is practically impossible to supply stably necessary power to each detonator because power lines for the crystal oscillator circuits will be blasted and lost by explosion of a detonator to be previously exploded. Further, the ordinary electric detonator utilizes two wires for supplying the electrical ignition energy to the detonator, and the detonator utilizing such crystal oscillator circuit which requires an additional wire for supplying power thereto will increase the wiring work cost and is not economical.
If the oscillation frequency of the crystal oscillator is increased to a higher level such as over several tens of MHz, it is true that the initial unstable period is shortened to several milliseconds. With an increased number of steps of frequency-divider circuit for counting the delay time, however, the integrated circuits making up the frequency-dividing circuit is increased.